1. Field of the Invention
This invention relates to alkali-resistant glass fibres and to cementitious products reinforced with such alkali-resistant glass fibres.
2. Description of the Prior Art
It is well known that glass fibres which are to be used as a reinforcing material for inorganic cements, such as ordinary Portland cement, must be resistant to attack in the alkaline environment of the cement matrix. Most commercially available alkali-resistant glass fibres are made from a relatively simple glass composition in which the components which act together to confer resistance to such attack are a combination of calcium oxide (CaO) and zirconium oxide (ZrO.sub.2). Considerable work has been reported in the literature, and in particular in the patent literature, on attempts to produce an improvement in performance over that of the available glasses. Efforts have also been directed to developing compositions whose performance in a cement matrix is comparable to that of the available glasses but which are also capable of being fiberised more economically. In order to obtain a fibre product which can be incorporated into a cement composite in a satisfactory manner, producing a reinforcing effect, it is necessary to make the glass fibre in continuous filament form by a mechanical drawing process, in which the glass filaments are drawn from multiple orifices in the base of a container known as a bushing. Such a process imposes limitations on the choice of suitable components to confer alkali resistance and the quantity of such components that can be used.
In order to draw continuous filaments in the equipment presently available, using bushings of a high temperature resistant platinum alloy, it is important that the actual drawing temperature should not exceed 1350.degree. C. and preferably not 1320.degree. C., as otherwise the useful working life of the bushings will be reduced with a resultant increase in production costs. It is also important that there should be a positive difference between the working or drawing temperature T.sub.w and the liquidus temperature T.sub.l of the glass. T.sub.w is defined as the temperature at which the viscosity is 1000 poises because this is the viscosity to which a glass melt is normally adjusted for mechanical drawing into fibres from a platinum alloy bushing. It is preferable to have a difference of at least 40.degree. C. between T.sub.w and T.sub.l and in practice bushing operators prefer a difference of about 80.degree. C.
Several materials have been identified as conferring alkali resistance on glass, but they generally also make the drawing of continuous filaments more difficult. ZrO.sub.2, for example, increases both the viscosity and the liquidus temperature of the molten glass. One cannot, therefore, simply keep adding more and more of any such material, particularly a material such as ZrO.sub.2, without causing the glass to have either a drawing temperature above 1350.degree. C. or a negative or insufficient value for T.sub.w -T.sub.l. Glass compositions of various formulations have been proposed in the prior art. British Pat. No. 1,290,528 (U.S. Pat. No. 3,861,926) in the name of the present applicant is believed to be the earliest specification which discusses the problem of obtaining a composition with adequate alkali resistance while still fulfilling the requirements for drawing temperature and T.sub.w -T.sub.l. Fibres having a composition within the scope of the claims of that patent are now commercially available under the Registered Trade Mark Cem-FIL. This composition is in weight %
______________________________________ SiO.sub.2 62 Na.sub.2 O 14.8 CaO 5.6 TiO.sub.2 0.1 ZrO.sub.2 16.7 Al.sub.2 O.sub.3 0.8 ______________________________________
U.S. Pat. No. 3,840,379 is an example of an attempt to formulate a composition with fiberising characteristics more closely resembling the commercially produced "E" glass (which is generally used where alkali resistance is not required) while still retaining an alkali resistance as good as the commercially available glass within the scope of British Pat. No. 1,290,528 whose composition is given above. The glasses of U.S. Pat. No. 3,840,379 contain TiO.sub.2 in addition to CaO and ZrO.sub.2. British Pat. Nos. 1,497,223, 1,540,770 and 1,548,776 in the name of Kanebo Limited, discuss the problem of obtaining fiberisable glass compositions with a content of ZrO.sub.2 of the order of 20 wt.%. Asahi Glass Co.'s British Patent No. 1,498,917 (U.S. Pat. No. 4,062,689) discloses glass compositions, said to have high alkali resistance and intended for making glass fibres for reinforcement of cementitious products, consisting essentially of:
______________________________________ wt. % ______________________________________ SiO.sub.2 45-65 ZrO.sub.2 10-20 Cr.sub.2 O.sub.3 0-5 SnO.sub.2 0-5 MO 0-18 M'.sub.2 O 0-18 SO.sub.3 0.05-1 ______________________________________
wherein the above components constitute at least 97 wt.% of the compositions, Cr.sub.2 O.sub.3 +SnO.sub.2 is 0.5-10 wt.%, ZrO.sub.2 +SnO.sub.2 +Cr.sub.2 O.sub.3 is 12-25 wt.%, M is one or more of Ca, Mg, Zn, Ba and Sr; M'.sub.2 O is one or more of K.sub.2 O (0-5 wt.%), Na.sub.2 O (0-18 wt.%) and Li.sub.2 O (0-5 wt.%). This specification thus discloses the use of compositions containing either or both of SnO.sub.2 and Cr.sub.2 O.sub.3 in addition to ZrO.sub.2, with a requirements for the presence of at least 0.5 wt.% Cr.sub.2 O.sub.3 when SnO.sub.2 is absent. There is a clearly stated preference for compositions containing SnO.sub.2, with or without Cr.sub.2 O.sub.3, with a total content of these materials in the range 1.5 to 5.5 wt.% and at least 1.0 wt.% SnO.sub.2. In the only example which contains Cr.sub.2 O.sub.3 and no SnO.sub.2, Cr.sub.2 O.sub.3 is 3% and ZrO.sub.2 is 20%. We find that a content of Cr.sub.2 O.sub.3 as high as 3%, with a ZrO.sub.2 content of 10% or higher, produces a glass with liquidus and viscosity characteristics which make the drawing of continuous filaments impossible.
Cr.sub.2 O.sub.3 is a well known component of glass compositions, being used to impart a green tint. Melting any glass batch composition containing, for example, sodium or potassium bichromate as a source of chromium produces a glass in which an equilibrium exists between trivalent and hexavalent chromium oxidation states. Methods of altering this equilibrium by providing either reducing or oxidising conditions are well known to glass makers (see Glass Industry April 1966 pages 200 to 203 "Conditions influencing the state of oxidation of chromium in soda-lime-silica glasses" and Bulletin of the American Ceramic Society Vol. 47 No. 3 (1968) page 244 to 247 "Color characteristics of U.V. absorbing emerald green glass"). The interest in this equilibrium has been primarily one of ensuring that the hexavalent ion is present in order to utilise its extremely strong absorption in the near ultra-violet spectrum.